It is very important for the efficiency and performance of the turbine part that the distance defining the clearance between the blade tips of the turbine disc and the stator housing of the turbine part, is as small as possible. This applies particularly to the continuous operating state in which the turbine is intended to be run. During start-up and load changes, the requirement for efficiency can be lowered.
The elements comprised by the turbine part, for example rotor shaft blades and stator housing, are heated and cooled differently rapidly during non-steady states, for example during start-up and load increases and during stop and load reductions. This is due to the fact that the elements have different mass and that they are influenced to a varying extent by the hot gas flow which passes through the turbine part. The heating of the elements results in linear expansion and deformations, which means that clearances between rotating and static elements during non-steady states are influenced.
During the non-steady states, the blade tip clearance is reduced and can be completely eliminated if, in cold or in heated condition, it is chosen too small. This leads to contact and seizure, which is unacceptable. To avoid contact between rotating and static parts, a clearance between the blade tips and the turbine housing is chosen which is sufficiently large to prevent blade tip contact during start, stop and load changes and which is sufficiently small during continuous operation to prevent an unacceptably low efficiency.
The clearance between the blade tips and the stator housing must thus be chosen on the basis of the operating state which gives the smallest permissible clearance taking into account the uneven temperature distribution, the extension of the blades because of the centrifugal force, and the like.
One way of reducing the blade tip clearance during continuous operation is to design the turbine such that expansion and deformation because of the temperature can be controlled by distributing the mass in the turbine such that movements and deformations are overcome or redistributed.
Another way is to introduce operating restrictions to avoid the most difficult operating states which are determining for the clearance between the blade tips and the stator housing.
Thus, the problem is to dimension the clearance between the blade tips and the stator housing so as to obtain the best possible performance and efficiency without the risk of blade tip contact with the stator housing arising, especially during start-up, stop and load changes, and without the clearance becoming unnecessarily large.